Take the 2-minute tour ×
Motor Vehicle Maintenance & Repair Stack Exchange is a question and answer site for mechanics and DIY enthusiast owners of cars, trucks, and motorcycles. It's 100% free, no registration required.

Earlier this year, I had my CEL come on on my '05 Mazda 3 due to P0128 (asked about here). I cleared the code, and it never came back. Six months later, I have the code P0126 come up and now I'm wondering if I should be replacing my thermostat or doing something else? And my follow up question will be - how do I actually do the replacement or other suggested task?

INFO: My engine temperature on my dashboard shows that it is below the range of temperatures when I start my car, and stays there until I start driving around 2 miles. It also doesn't get to the middle of the temperature range until I am at highway speeds.

share|improve this question

3 Answers 3

You should think of the temperature gauge on the dash as more of an indication of the engines temperature rather then the engines actual temperature. Some generic OEM ECU programme maps will replace a faulty temperature reading with a substitute value to operate the vehicle and in some/most cases you would not notice the difference. Use an OBD2 scanner to see the reading from your engine temperature sensor, OBD2 readings are the actual readings of the sensors and are never substituted, generic/OEM are occasionly substituted. Checking the engine with an infra red thermometer is also a good check to confirm or cross reference temperatures. Also with an OBD2 scanner check that all the monitors have run. You need to run a drive cycle for the vehicle if they are not all set to completed. If there is a secondry DTC present then a lot of earlier ECU drive cycle programmes will not run at all and you could drive around indefinitely with a fault which would not turn on the check light. A drive cycle must be run exactly, even having the radio on or off can effect the running of earlier set ups. Your thermostat may be electrically heated, check for code P0597/0598/0599. Check the sensors and monitors with OBD2 first.

share|improve this answer
    
You clearly know more than I do about cars, so I want to ask some clarifying questions. What is an ECU program? I used my OBD2 scanner and the only codes coming up are P0126 and now P0128 (its back). The only thing my OBD2 Scanner (amzn.to/1hDH1FS) certain monitors are complete, nothing about temps. I don't have an IR Thermometer. What does it mean to run a drive cycle? How do I do that? What is a Secondary DTC? Thanks for the help so far! –  tarheel Dec 7 '13 at 6:09
    
In deciding how to give a credible answer to additional questions I asked a friend. The answer we came up with is to recommemd a book, "OBDII Functions, Monitors, and Diagnostic Techniques - by Al Santini" This book has been described to me as a little gem. The tool you have is not a scanner. It is a code reader. A code reader offers only the minimum of information ie the code. The code should also have a descriptor. A scanner will display information from OBDII modes, explained fully in the book. –  Allan Osborne Dec 7 '13 at 10:00

If all else fails check to see your thermostat is opening and closing at correct temps. all you need is hot water and a thermometer.Workshop manual particular to your car explains how.Dead easy

share|improve this answer

If you drop by your local plumbing shop and ask them for a "test thermometer" with a range up to 212*F, they'll sell you a neat little tool to keep in your toolbox. It'll have a fully round dial with a 6" steel probe sticking out of its back. The tip of the probe is the sensitive spot.

It's a fully mechanical device, never needs batteries, has no electronics to die, and may last past the end of your own life.

When it doubt about something like this, a test thermometer is the low-tech (reliable) way to determine temperature - insert the probe tip into a bolt hole on the intake manifold, or electrical-tape it to the upper radiator hose, or push it firmly (but not enough to damage things) between a pair of radiator fins near the upper radiator hose. It'll tell you what your temperature really IS.

Given your history, including the two OBDII codes and also given that your temp gauge is so slow to rise, and further given that a thermostat is a simple & cheap thing to replace... I'd suggest you swap it out with a known good thermostat and see what glints up from your goldpan.

share|improve this answer

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.